How to get absolute path with proper character encoding in Java?

The URL.getFile call you are using returns the file part of a URL encoded according to the URL encoding rules. You need to decode the string using URLDecoder before giving it to File:

String path = Swagger2MarkupConverterTest.class.getResource(
        "/json/swagger.json").getFile();

path = URLDecoder.decode(path, "UTF-8");

File file = new File(path);